1. Field of the Invention
The invention relates to a rotor for a rotary electric machine, and more particularly to a rotor for a rotary electric machine in which a plurality of magnetic poles are disposed at intervals, in a circumferential direction, at an outer periphery of a rotor core.
2. Description of Related Art
For example, Japanese Patent Application Publication No. 2008-306849 (JP-A-2008-306849) discloses a rotary electric machine that has a stator in which a stator coil is distributedly wound around an inner periphery portion, and a buried permanent magnet-type rotor that is rotatably provided in the stator. The above rotor is made up of a rotary shaft and a cylindrical core body that is fixed to the rotary shaft.
The above core body results from integrally constructing, by crimping or the like, an axial-direction stack of magnetic steel sheets, each formed through punching, into a circular ring-like shape. A plurality of magnetic poles, evenly spaced in a circumferential direction, are provided in the outer periphery of the core body. FIG. 8 illustrates one magnetic pole in a state viewed from an axial-direction end face. FIG. 8 illustrates one magnetic pole 104, together with part of a stator 106, from among the members that are disposed evenly spaced (45°-angle intervals such that the central axis of a rotary shaft is the center of each fan shape) on the outer periphery of the core body 102 of the rotor 100.
A plurality of teeth 108 that point inward in a radial direction are provided, at equal spacings in the circumferential direction, on the inner periphery of the stator 106. Slots 108 are respectively formed, in a number identical to that of the teeth 106, between mutually adjacent teeth, such that the slots 108 are opened on the inner periphery side and at both end portions in the axial direction. A stator coil (not shown) that is wound around the teeth 106 is inserted into the slots 108. As a result, a rotating magnetic field is formed on the inner periphery side of the stator 100 when the stator coil is energized.
Each magnetic pole 104 provided in the core body 102 of the rotor 100 is made up of three permanent magnets, namely permanent magnets 112, 114, 116. The permanent magnet 112 disposed in the circumferential direction center of the magnetic pole 104 is buried in the vicinity of an outer peripheral face 103 of the core body 102. The permanent magnet 112 has end faces and a cross-section that exhibit an elongated rectangular shape, and is formed to substantially the same axial-direction length as that of the core body 102. The permanent magnet 112 is disposed in such a manner that the longitudinal direction thereof, on the magnet end face, runs substantially along the outer peripheral face 103 of the core body 102, and has a longitudinal-direction width W.
The other two permanent magnets 114, 116 are disposed symmetrically on both sides of the permanent magnet 112, in the circumferential direction, such that the two permanent magnets 114, 116 form a V-shape that widens towards the outer periphery. In other words, the permanent magnets 114, 116 are disposed in such a manner that the distance or mutual spacing becomes narrower towards the inner periphery, and the spacing between inner periphery-side end portions of the permanent magnets 114, 116, at which the mutual spacing is the narrowest, is narrower than the longitudinal-direction width W. In the magnetic pole 104, as a result, a substantially triangular magnetic path region 118 is formed or defined by the three permanent magnets 112, 114, 116. Both end portions of the magnetic path region 118, in the circumferential direction, are linked to the outer peripheral face 103 of the core body 102 via regions between the permanent magnet 112 and the permanent magnets 114, 116.
JP-A-2008-306849 indicates that in a rotary electric machine provided with a rotor 100 having the above configuration, it becomes possible to reduce counter electromotive force of a particular order that is generated upon operation of the rotary electric machine, and noise as well can be reduced, by setting, to a predefined angle, the intersection angle that is defined by virtual straight lines that join the circumferential-direction end portions of the permanent magnets 114, 116 and the center of the rotary shaft, and a virtual reference line that runs through the center of the rotary shaft and that is perpendicular to a radial-direction straight line that runs through the circumferential direction center of the permanent magnet 112.
In the rotary electric machine of JP-A-2008-306849, magnetic flux flow such as those illustrated in FIGS. 9A to 9C is formed at the magnetic pole 104 of the core body 102 of the rotor 100 upon rotational driving of the rotor 100 as a result of flow of current through the stator coil. FIG. 9A schematically illustrates the flow of magnetic flux (hereafter, magnet magnetic flux), generated by the permanent magnet 114, towards the outer periphery through a magnetic path region 118. FIG. 9B schematically illustrates the way in which magnetic flux, which is generated by a q-axis current component obtained by resolving the vector representing the electric current flowing through the stator coil, on a d-q plane that is a Cartesian coordinate system (hereafter, q-axis current magnetic flux or excitation current magnetic flux), flows from the inner periphery end portions of the teeth 108 of the stator 106 into the core body 102 and traverses the magnetic path region 118 in the magnetic pole 104. FIG. 9C schematically illustrates the flow of magnetic flux resulting from combining the abovementioned magnet magnetic flux and the abovementioned q-axis current magnetic flux.
With reference to FIG. 9A, the magnet magnetic flux generated by the permanent magnet 114 is directed towards the outer peripheral face 103 of the core body 102. Part of the magnetic flux coming out of the permanent magnet 114 is directed towards the outer peripheral face, through the permanent magnet 112. However, the permanent magnet 112 that is buried in the core body 102 has a magnetoresistance or relative permeability that is equivalent to that of a void, and hence most of the magnetic flux flows towards the outer peripheral face, through the circumferential-direction end portion region of the magnetic path region 118, which is a steel sheet portion having low magnetoresistance. With reference to FIG. 9B, the q-axis current magnetic flux that flows into the magnetic pole 104 in the core body 102 is likewise directed towards the outer peripheral face, by flowing substantially along a circular arc through the magnetic path region 118 having low magnetoresistance.
Consequently, upon overlap of the magnet magnetic flux and the q-axis current magnetic flux that flow as described above, the density of the resultant magnetic flux increases at a substantially triangular downstream region 120, denoted by a hatched portion within the magnetic path region 118, as illustrated in FIG. 9C, and as a result, magnetic saturation is likelier to occur. In turn, this can result in a lower torque of the rotary electric machine.
In FIG. 9A, the magnetic flux generated by the permanent magnet 116 is not depicted. However, the magnetic flux from the permanent magnet 116 flows also towards the abovementioned downstream region 120, and hence there increases the possibility of magnetic saturation such as the above-described one. If the flow direction of the magnetic flux in the magnetic path region 118 is reversed, occurrence of magnetic saturation as described above is still likely at a region positioned between the permanent magnet 116 and the permanent magnet 112, within the magnetic path region 118.